1. Field of the Invention
The present invention relates to radios and global positioning system (GPS) receivers. More particularly, the invention relates to a combined GPS receiver and radio unit operable for sending GPS-derived data and other information to other such units.
2. Description of the Prior Art
The global positioning system (GPS) is an electronic satellite navigation system which permits users to determine their position with respect to the Earth. Global positioning may be determined with a handheld GPS receiver which detects and decodes signals from a number of satellites orbiting the Earth. The signals from each of these satellites indicate the position of the satellite and the time at which the signals were sent. To decode the satellite signals, known as spread spectrum signals, and thereby calculate desired position and navigational data, a GPS receiver must first “find” or acquire the signals emitted from a minimum number of satellites. Once the receiver is “locked on” to the spread spectrum signals, the user's global position and other navigational data (e.g., velocity of movement) can be calculated.
In addition to the growing popularity and demand for GPS navigational devices, portable two-way radios continue to enjoy widespread popularity. In 1996, the Federal Communications Commission (FCC) established the Family Radio Service (FRS) so that families and other small groups could communicate with one another over very short distances (typically less than two miles) at no charge and without an FCC license. The FRS, which is one of the Citizens Band Radio Services, includes 14 channels in the UHF 465 MHz band. There are a number of commercially available two-way radios which are designed for use with the FRS.
The increasing use of both portable radios and GPS receivers has led to proposals to incorporate the two into combined units. It is anticipated that such combined units will provide enhanced capabilities such as the ability to transmit GPS-derived location data between two or more units so that the users of the units may monitor each other's location. For example, a family may use several of these combined units to not only communicate with one another but also to monitor each other's location so that parents may keep track of their children.
Unfortunately, currently-proposed combined GPS/radio units suffer from several limitations that limit their utility. For example, proposed designs for combined GPS/radio units include, in addition to the typical controls found on GPS receivers and FRS radios, numerous other buttons that must be operated to trigger the transmission of location data between the units. Those skilled in the art will appreciate that such additional buttons complicate operation and increase costs of the units.
Another limitation of currently-proposed combined GPS/radio units is that location data is only transmitted from a first unit to other units when the user of the first unit elects to do so. This is a problem because users often wish to know the location of other users, even if these other users forget to transmit such information. For example, parents may wish to monitor the location of their children, but only may do so with currently-proposed units if the children periodically transmit their location information back to their parents.
Another limitation of currently-proposed combined GPS/radio units and FRS radios in general is their lack of privacy. Because the FRS is an unlicensed, shared communication service, communications from several different groups of people are often transmitted at the same time, causing interference. This is especially true in areas where FRS radio use is popular, such as in amusement parks, sporting events, concerts, etc. Such interference can be avoided to a certain extent by selecting between 14 available FRS channels, but often, some or all of the channels are used simultaneously. Also, once members of a group have selected a channel and separated from one another, it is difficult to get all of the members to simultaneously switch to a different channel.
To prevent FRS radio users from hearing unwanted radio transmissions, many FRS radios incorporate continuous tone coded squelch systems (CTCSS). A CTCSS controls the audio output of a radio so that users only hear desired communications. A CTCSS includes circuitry for generating and a selector for selecting between a number of sub-audible tones, one of which is transmitted any time a radio is operating in the CTCSS mode. Other radios with a CTCSS that receive the transmission will disable their squelch and therefore enable audio output if they have been set to the same sub-audible tone. The radios will not disable their squelch for communications that are transmitted with a different or no CTCSS sub-audible tone.
Unfortunately, many people incorrectly assume that a CTCSS prevents others from listening to their conversations. In fact, a CTCSS only prevents a user from hearing unwanted conversations, but does not prevent others from listening to the user's conversations. Radios that do not have a CTCSS or that have a CTCSS that is turned off can receive and hear all communications transmitted in their vicinity. For example, a group of users may enable the CTCSS on their radios and communicate with one another without hearing communications from other people. However, users of other radios without a CTCSS or with a CTCSS that is not enabled may listen to all of groups' communications. The same is true for transmitted location data. For example, with prior art combined GPS/radio units, unauthorized persons may intercept transmitted location data and use such data to improperly monitor the location of other radio users.
Another limitation of currently-proposed combined GPS/radio units and FRS radios in general is that it is sometimes difficult to transmit and receive emergency messages. FRS rules require users to give priority to emergency messages; however, FRS channels are frequently so crowded that emergency messages cannot be heard or are interfered with. Moreover, users of radios with a CTCSS as described above cannot hear an emergency message unless the CTCSS on their radios is turned off or tuned into the same sub-audible tone transmitted with the emergency message.
Accordingly, there is a need for an improved combined GPS/radio unit that overcomes the limitations of the prior art.